Bony pelvis

By

Dr Manah Chandra Changmai MBBS MS

Derived from latin word means Basin

Ring of bone:Two hip boneSacro-coccygeal part of vertebral column

The pubic part of hip bone connected by pubicSymphysis.

Introduction

Pelvic bone is made up of various sections:

For obstetrical purposes, the pelvis is divided by thepelvic brim into two parts:– The False Pelvis– The True Pelvis

The False Pelvis is that portion above the pelvic brim. It does nottake part in the mechanism of delivery and is of no obstetricinterest.

In the past attempts were made to form a judgement of the size ofthe true pelvis by measuring the width of the false pelvis. Theinformation thus obtained was often inaccurate

False Pelvis

Intercristal diameter [IC ~29 cm]:widest point on lateral aspect of iliaccrest

Interspinous diameter [IS ~26 cm]:distance between the lateral tips ofthe anterior superior iliac spines

External conjugate [AP] diameter[EC ~20 cm]: distance between apexof spine of 5th lumbar vertebra andcentre of the superior border ofsymphysis pubis.

False Pelvis

The True Pelvis is that portion below the pelvic brim. Itdetermines the size and shape of the birth canal.

Pelvic Brim or Pelvic inlet: formed by the uppermargins of pubic bones, theilio-pectineal lines and theanterior upper margin of thesacrum.

Cavity: formed by the pubicbones, ischium, ilium, andsacrum

Outlet: diamond-shapedmade up of the pubic bones,ischium, ischial tuberosities,sacrotuberous ligament, and5th segment of sacrum

True Pelvis

Pelvic inlet

Pelvic inlet is formed from behind forward by

a. Sacral promontory

b. Anterior margins of ala of the sacrum

c. Linea terminalis

d. Upper end of symphysis pubis.

A-P diameter or anatomical conjugateExtends from middle of sacral promontoryTo the upper margin of symphysis pubis.

Oblique diameter:Sacroiliac joint of one side to the iliopubicEminence of other side.

Transverse diameter:Widest of all the diameters.

Pelvic inlet

Pelvic cavity

Extends downwards and backwards from pelvic inlet,intervenes between inlet and outlet.

Posterior wall of the cavity longer than anterior wall.

Boundaries

AnteriorlyBy symphysis pubis and body of the pubis with its rami

Posteriorly Concave pelvic surface of sacrum and coccyx.

On each sideQuadrangular area formed by pelvic surface of ilium and ischium.

Pelvic cavity

Anterior posterior diameter:From middle of the back of symphysispubis to the pelvic surface of third sacralvertebrae.

Oblique diameter:Lower end of sacroiliac joint to the centre ofobturator membrane.

Transverse diameter:Across the lateral bony walls of pelvic cavity.

It is diamond shaped and wider in female.

Boundaries

In frontLower margin of symphysis pubis connected by arcuate pubic ligament.

Behind Tip of the coccyx

AnterolaterallyConjoint ischiopubic rami.

LaterallyIschial tuberosities.

PosterolaterallyThe sacrotuberous ligament.

Pelvic outlet

Pelvic outlet ( inferior view )

Pelvic outlet

Anterior –posterior diameter:from lower border of symphysis pubisto tip of the coccyx.

Oblique diameter:Between the junction of ischio-pubic ramusof one side and middle of the sacrotuberousligament of the opposite side.

Transverse diameter:between the medial surfaces of the lower endsof ischial tuberosities.

Types of pelvises

Gynaecoid

Anthrapoid

Android

Platypelloid

Gynaecoid pelvis

Ideal pelvis favouring a normal delivery; 50.6% of women

Brim slightly ovaltransversely but almostRounded.

Sacrum curved Ischial spines notprominent

Short-cone pelvis Obtuse greater sciatic notch

Triangular obturatorforamen

Sub-pubic arch rounded[Roman arch] angle at least900

Male-type pelvis favouring OP positions and apt to cause deep transversearrest of head; 22.4% of women.

Brim heart-shaped

Sacrum curved

Ischial spines prominent

Long-cone funnel pelvis

Acute greater sciatic notch

Oval obturator foramen

Sub-pubic arch very narrow[Gothic arch]

Android pelvis

Ape-like pelvis favouring OP positions often requiring operative vaginaldeliveries; 22.7% of women.

Brim AP oval Sacrum very slightly curved

Ischial spines prominent

Long-cone funnel pelvis with straight sidewalls Obtuse greater sciatic notch

Oval obturator foramenSub-pubic arch narrow

Anthrapoid pelvis

Platypelloid pelvis

Brim oval transversely

Sacrum very slightly curved

Ischial spines prominent

Short-cone shallow pelvis

Acute greater sciatic notch

Triangular obturator foramen

Wide arch narrow

Leads to cephalo-pelvic disproportion; 4.4% of women.

Asymmetrical pelvises

MALE FEMALE

Bone are taller, heavier, and thicker Bones are lighter and less dense

Illiac fossa is more concave and the anterior superior iliac spine is inturned.

Illiac fossa is shallow and anterior superior illiac spine is straight forward.

Acetabular cavity is large. Acetabualr cavity is shallow.

Obturator foramen is large and oval. Obturator foeamen is small and triangular.

Pelvic inlet is heart shaped Round in shaped and diameters are longer than male.

Pelvic cavity longer and more conical Pelvic cavity shorter and more conical.

Symphysis pubis and body of the pubis are elongated. Symphysis pubis and body are short.

Pelvic outlet is small and 1” lesser the the diameters of female.

Pelvic outlet is wide and anterior posterior diameter is longest.

Sub-pubic angle is narrow and measures 50- 60 Sub-pubic angle is wide and measures about 80-90.

Angle and depth of greater sciatic notch are narrow. Angle and greater sciatic notch are wide.

Ischial spine are inturned Ischial spines are out turned.

Curvature of the pelvic surface of sacrom is uniformly concave.

The upper part is more flat and lower part is abruptly concave.

Clinical Assessment Body build

Engagement defined as the point when the engaging diameter [BPD(biparietal diameter = ~10 cm] goes past the pelvic brim.Five fingers = 10 cm.

Fifths palpable above symphysis pubis

Clinical Assessment foetal head as pelvimeter

In Gynaecoid & Android pelvisdistance between ischial spine to brimis ~5 cm.

In Anthropoid pelvis distance is ~7cm

In Platypelloid pelvis distance is ~3cm

Station of the head inrelation to ischial spines

Clinical Assessment foetal head as pelvimeter

vaginal examination

Clinical Assessment vaginal examination

vaginal examination

Presentation of the fetus at the time of delivery

1. True AP Conjugate2. Obstetric Conjugate3. Mid-cavity AP Conjugate4. Outlet AP conjugate5. Angle Greater Sciatic notch6. Angle of inclination of pelvic brim7. Angle of inclination of sacrum8. Ischial spine9. Ischio-tuberous distance10. Foetal head lie, position, engagement

Clinical Assessment radiological examination

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